Conventionally, for example, in a video camera using a solid-state image pickup device, when the signal which was read out is clamped, in many cases, a light-shielded part is provided in an image pickup cell corresponding to the back porch in the horizontal flyback interval to obtain a stable optical black reference signal in each horizontal flyback interval. This is quite obvious in consideration of the points that the dark current which is a factor of variation in black level of the solid-state image pickup device is approximately doubled whenever the temperature of the device is increased by 8.degree. C. and that the ordinary conditions in which the video camera is used lie within a range from -10.degree. C. to +40.degree. C. To accomplish the above-mentioned object, in general, by light-shielding a few to several tens of image pickup cells corresponding to the back porch in the horizontal flyback interval of the solid-state image pickup device, only the dark current component is formed, thereby clamping the signal level in this portion to a reference black level. In order to perform the stable clamping operation in this case, it is desirable that the number of image pickup cells which are allocated to the light-shielded part be large. However, there is a drawback that the use of cells in the light-shielded part causes the resolution to be further reduced in the situation that there are few or no image pickup cells to spare in the horizontal direction.
On the other hand, an increase in number of image pickup cells in a device of a given size implies a large area and high integration for the device, so that this is a difficult problem unless processing technology is remarkably improved.
As a method of solving such a problem, a clamping method of the feedback type is generally known in video cameras using an image pickup tube. Namely, this method is one in which in a system comprising a clamp circuit to clamp a level in the beam blanking interval as a black level to a reference level and a processing circuit to amplify this clamped signal and thereafter to perform the .gamma. correction and the like, an output of the processing circuit is compared with a predetermined reference level in order to make an output of this system stable, and a feedback circuit is provided for the above-mentioned clamp level in order to set the differential output therebetween to be zero.
However, in the case of employing this method for, in particular, the solid-state image pickup device, the noise in the blanking interval can be easily mixed since an output amplifier in such a solid-state image pickup device has a floating structure or the like.
In addition, the reset potential varies due to the temperature characteristic and difference in saturated resistance of a switching transistor in association with the resetting operation to the reference potential when the signal is read out; consequently, the blanking interval cannot be used for the signal which is clamped upon clamping in this state.
On the other hand, in solid-state image pickup devices, clock noise is generally generated (approximately one-third of the saturation signal level) due to the capacity coupling of the signal readout shift pulse in the signal readout interval. Therefore, when the potential at the light-shielded part including this clock noise is detected and is compared with a certain clamping reference level as in the conventional manner, a problem is caused such that the DC component of the video signal cannot be reproduced with a high degree of accuracy.
As a method of solving this problem, a method is known whereby sample and hold means to remove the above-mentioned clock noise is provided at the front stage of the feedback clamp circuit. However, the clock noise of the command pulse of tens of millivolts is generated even in the sample and hold means when considering realization of low voltage and low electric power consumption. On one hand, as the signal level, a level of the order of about 1/3 to 1/8 of the saturation signal output level of the solid-state image pickup device is used as the standard operating level. However, since the saturation signal output level of the solid-state image pickup device is ordinarily hundreds of millivolts, the signal level is a value of the order of approximately hundreds of millivolts. Therefore, the ratio of the above-mentioned clock noise to this signal level is about 20 dB, so that it is necessary to set the performahce of the feedback clamp circuit to be less than about -30 dB in order to set the black level stability (or low-frequency conversion noise of the clock noise) to be less than about - 50 dB, which value is ordinarily required for the image pickup apparatus.
In addition, in the case of using the above-mentioned solid-state image pickup apparatus as the electronic still camera to pick up a still picture image, low electric power consumption and good leading characteristic after power-on are required. In other words, after a release switch (power switch) is turned on, the electric circuits have to be capable of immediately processing the signal from the solid-state image pickup device. For this purpose, it is desirable to directly couple the electric circuits instead of coupling them through capacitors.
However, in the case of directly coupling them, since an output amplifier of the solid-state image pickup device is constituted by an MOS amplifier and there is a variation in threshold voltage of the MOS, the apparatus has to be constituted in a manner such that a sample and hold circuit having a small dynamic range is provided immediately at the post stage of the solid-state image pickup device and thereafter the signal is amplified and is input to a clamp circuit. With such an arrangement, since the clock noise is also amplified, further potential stability is needed in the clamp circuit.